A) Field of the Invention
The present invention relates to a manufacture method for a solid state imager whose pixel in a light reception area has a plurality of photoelectric conversion elements (photodiodes) having different characteristics.
B) Description of the Related Art
Various solid state imagers are known such as a charge-coupled device (CCD) solid state imager which transfers signal charges accumulated in a photosensitive element by using a CCD and a MOS type solid state imager which amplifies and outputs signal charges accumulated in a photosensitive element (converts the signal charges into voltage or current). Photodiodes are mainly used as the photosensitive elements and a number of pixels are disposed in a matrix shape in a light reception area. As the layout of photosensitive elements, there are a tetragonal matrix layout which disposes photosensitive elements in a tetragonal matrix shape at a constant pitch both in the row and column directions and a honeycomb layout which disposes photosensitive elements by shifting the element positions every second row and column directions (e.g., by shifting by a half pitch).
A number of photodiodes can be formed, for example, by regularly forming a number of n-type charge accumulation regions in a p-type well. Each n-type charge accumulation area accumulates electrons photoelectrically converted therein. The amount of charges capable of being accumulated by each n-type charge accumulation area has a maximum value (saturated charge amount), and cannot accumulate charges more than the maximum value. Charges larger than a predetermined amount (minimum charge amount) is required in order to form an output signal larger than a noise level. Namely, the amount of signal charges capable of being accumulated by one photodiode has both the maximum and minimum values whose corresponding incidence light amounts define an optical dynamic range.
The optical dynamic range of each photodiode is apparently narrower than that of a silver salt film. If the incidence light amount per photodiode is increased to raise the sensitivity and photograph a dark image, a large amount of signal charges are generated for a bright object and exceeds the saturated charge amount. For example, information of the details or others of a white wedding dress is lost. If the sensitivity is lowered to record the details of a bright object, a dark object cannot be photographed.
High sensitivity image information and low sensitivity image information at the same position (pixel) may be used to realize a broad optical dynamic range of a solid state imager using photodiodes. Imaging in long and short exposure times provides image information of two types, high and low sensitivities. The optical dynamic range can be broadened by synthesizing the two types of image signals. However, since the exposure timings of two types of imaging are displaced, some problems occur for a moving object.
Japanese Patent Laid-open Publication No. 2000-125209 proposes that at the inter-lattice positions of a first tetragonal matrix of high sensitivity light receiving elements having a large area, a second tetragonal matrix of low sensitivity light receiving elements having a small area is disposed to constitute a honeycomb matrix as a whole, and the two types of these light receiving elements receive color-separated transmission light to thereby broaden the dynamic range. In this case, the first and second tetragonal matrices are independently provided with a charge read/transfer vertical CCD.